| Summary: | Three previously unreported photo-carrier relaxation pathways are presented and discussed in GaAs-based systems. In bulk GaAs, a transient bleach of the spin-split exciton transition 1s->2p is reported following photo-excitation at low temperatures and is likely caused by final state blocking of the 2p_1 exciton level. The bleach of the 1s->2p_-1 transition is delayed with respect to that of the free carriers and 1s->2p_+1, suggesting electronic relaxation occurs through two simultaneous mechanisms: elastic scattering between quantized conduction band states and spin-dependent relaxation through the 2p_1 exciton states. For ErAs:GaAs composites, the response at short time delays is completely dependent on the occupation of the interface trap state between the ErAs nanoparticles and the GaAs matrix. Occupation of the interface state depends on the photo-carrier energy, carrier density, and trap density. Carrier scattering from the interface state plays a large role in the response as it prevents full relaxation of the system on ultrashort timescales. The composite ErAs:GaAs systems also exhibit an oscillatory response highly suggestive of surface plasmon polaritons at the interface between the semi-metallic ErAs and semiconducting GaAs, which couple to the GaAs phonon modes. The oscillation frequencies are observed to follow the same trend with volume fraction as the static absorption resonance peaks, suggesting different nanoparticle size distributions exist with different ErAs incorporation.
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